1. Field of the Invention
The present invention concerns an annular air flow passage, particularly for a turbine engine, such as a turbofan or turboprop, comprising an elongated element passing through the flow passage and capable of being instrumented.
2. Description of the Related Art
Conventionally, a dual-flow turbofan 10, as illustrated in FIG. 1, consists of a gas turbine 12 with a revolution axis 14 driving a ducted fan wheel 16, wherein the latter is generally positioned upstream from the turbofan. The mass of air sucked in by the engine is divided into a primary air flow (arrow A) that flows through the gas turbine 12 or engine core and a secondary air flow (arrow B) originating from the fan 16 and surrounding the engine core, wherein the primary and secondary air flows are concentric and circulate in a primary annular flow passage 18 and a secondary annular flow passage 20 respectively 8.
In a manner well known per se, the primary air flow (arrow A) is generally compressed by a low-pressure compressor 22 and subsequently by a high-pressure compressor 24, each having vanes both fixed 26 and mobile arranged alternatively in the direction of movement of the flow. The low-pressure compressor shaft is connected to the fan wheel 4 and is driven in rotation by the shaft of a low-pressure turbine arranged downstream (not illustrated). The low-pressure compressor shaft is driven in rotation by the shaft of a high-pressure turbine arranged at the outlet of a combustion chamber and located upstream from the low-pressure turbine (both not illustrated).
In a double-body turbofan of this kind, fan casing usually designates the external annular wall 28 surrounding the fan wheel 16 and intermediate casing 30 designates a structural element of the turbine engine interposed axially between the compressors, low-pressure 22 and high-pressure 24, which passes through the annular flow passages, primary 18 and secondary 20. This intermediate casing 30 comprises two annular walls, radially internal 32 and external 34, respectively delimiting, internally and externally, the primary annular air flow passage 18 and two annular walls, radially internal 36 and external 38 delimiting internally and externally the secondary annular flow passage 20, respectively.
Within the context of developing a turbofan, the latter's performances need to be tested with a view to its certification. Development turbine engines are therefore provided for this purpose. A large number of measurements are performed on these turbofans. The characteristics of the aerodynamic flow in particular are measured at specific axial positions or measurement planes 40a, 40b, 40c. 
For this purpose, measuring elements 42a, 42b, 42c, commonly known as measurement sensors, arranged on the measurement planes 40a, 40b, 40c, are generally used to characterise the aerodynamic flow by measuring parameters such as pressure and temperature for example during operation. Such an element, 42a, 42b, 42c, comprises a first radially external end 44 and a second end 46, radially internal in relation to the axis of rotation. As shown in FIG. 2, the first end 44 of the element 42a comprises a base 48 fixed by bolting to the fan casing 28. The base 48 is thus fixed rigidly in all directions to the wall 28.
The element may extend appreciably in a radial direction like elements 42a or 42b, which are arranged in the secondary air flow and are rigidly fixed by their radially external end or like element 42c, which extends appreciably perpendicularly from the internal wall 32 internally delimiting the primary air flow passage.
The second end 46 of the element opposite the first end 44 fixed to the turbine engine is generally free, i.e. with degrees of freedom in the axial, radial and circumferential directions.
These elements 42a, 42b, 42c, are termed intrusive, since immersed in the primary or secondary air flow. The measuring element 42a comprises a tubular body 50 with an external aerodynamic shape liable to affect as little as possible passage of a flow of air. The body 50 comprises an upstream surface 52 provided with holes distributed along the direction of elongation of the body 50. In the embodiment shown in FIG. 2, a cylindrical nozzle 54 is installed in each hole so as to protrude in the upstream direction in relation to the upstream surface 52. Each nozzle 54 is equipped with means of measurement of characteristics of a flow, such as temperature or pressure for example.
Owing to their intrusive nature in the aerodynamic flow passages of the engine, a study of the vibration behaviour of the instrumented elements is performed systematically during the design phase. It is therefore important to limit resonance phenomena of the element liable to cause cracks in the measuring element capable of affecting its mechanical integrity. In extreme cases, formation of nicks or cracks as a result of the vibrations may cause partial or total dislocation of the element 42a, 42b, 42c. The debris thus released circulates in the flow passage and may damage components of the turbine engine arranged downstream. It is clear that the damage caused by such dislocation may be particularly severe when a measuring element 42a, 42b, 42c is installed in the primary flow passage, since the debris pay damage the combustion chamber and the fixed and rotating components of the high-pressure and low-pressure turbines.
This resonance phenomenon of the element may be due to several sources of vibratory stimuli within the turbine engine. A first source of vibration results for example from the residual imbalance of the rotating assemblies, i.e. of the low-pressure and high-pressure rotors. A second source of vibration originates from the alternation of the compression and decompression phases due to rotation of a row of mobile blades. This second source of vibration proves particularly intense when the measuring element is arranged immediately downstream from an impeller as is the case with the element 42a in FIG. 1.
By way of an example, a fan wheel, comprising 30 blades, revolving at a rotation speed of 2000 rpm, generates a pulsation of air in an axial direction of around 1000 Hz. If the first normal mode of the measuring element is close to 1000 Hz, the element will have a high risk of resonating in this case.
A measuring element has natural frequencies that are fixed and depend on its structural and dimensional characteristics. When the vibration frequency of the element f1 comes close to its resonance frequency fr1 of rank 1 or its harmonic natural frequencies, there is a high risk of resonance of the measuring element, which increases the risk of crack formation.
For purpose of clarity, we will take as an example of natural frequency the resonance frequency fr1 of rank 1.
In order to minimize resonance phenomena, the engine operating ranges should be limited in this case to ranges in which the vibration frequency f1 is sufficiently distanced from the resonance frequency fr1. In other words, some ranges of operating speeds of the turbofan may be prohibited in the presence of the measuring element, thereby reducing the value of the engine trials. Stoppage of the test turbine engine may therefore be necessary in order to change the element, which results in an increase in costs.